1. Field of the Invention
The invention relates to a process for producing monolithic oxidation catalysts and to their use in the gas-phase oxidation of hydrocarbons.
2. The Prior Art
Supported catalysts for the gas-phase oxidation of hydrocarbons to give the corresponding oxidation products such as carboxylic acids, carboxylic anhydrides or aldehydes, which catalysts have a catalytically active surface coating consisting essentially of titanium dioxide (TiO2) and divanadium pentoxide (V2O5), have been known for a long time. A typical example of the use of such catalysts is the preparation of phthalic anhydride, in which mixtures of o-xylene and air or naphthalene and air or o-xylene, naphthalene and air are passed over an appropriate catalyst in a shell-and-tube reactor. The heat generated in this strongly exothermic reaction is customarily removed (cooling, isothermal reaction conditions) by means of a salt melt which surrounds the reaction tubes.
The supported catalysts used here comprise an inert support body, for example having a ring shape or a spherical shape, on which the actual catalytically active composition is present. The active composition consists predominantly of the main components TiO2 in the anatase form and V2O5. To improve the control of the activity and to improve the selectivity, further activating or deactivating additives, for example oxides of transition elements or alkali metal compounds, are frequently added in small amounts as dopants (promoters) to the catalytically active composition.
The supported catalysts are generally produced by spraying aqueous suspensions or aqueous solutions of TiO2 and V2O5, frequently with addition of promoters and possibly binders for improving adhesion of the active components to the support, onto the support bodies.
As support bodies, use is made of regularly shaped, mechanically stable bodies such as spheres, rings, half rings, saddles, etc., particularly preferably rings or spheres. The size of the support bodies is determined predominantly by the dimensions of the reactor, especially by the internal diameter of the reaction tubes.
Support materials used are, for example, steatite, Duranit, earthenware, silicon dioxide, silicon carbide, aluminates, metals and metal alloys.
EP-A 744214 (U.S. Pat. No. 5,792,719) discloses a procedure for producing catalysts in which TiO2, V2O5, SiC and possibly dopants such as CsCO3 and (NH4)2HPO4 are stirred in aqueous suspension for a number of hours, and the suspension is subsequently admixed with an organic binder. This suspension is sprayed onto the support material and the supported catalyst is dried.
In industry, it is customary for each of the reaction tubes to be filled with various catalysts which have different catalytically active compositions. These can be arranged, for example, in two superposed zones, an upper zone and a lower zone. This measure makes it possible to match the activity of the catalyst system in the reactor appropriately to the course of the reaction.
During the reaction, the major part of hydrocarbon is reacted in the upper part of the reaction tube. As a result, the highest temperatures inevitably also occur there. In the lower part of the tube, only a sort of after-reaction takes place. There, remaining o-xylene/naphthalene and intermediates, for example o-tolualdehyde and phthalide, are converted into phthalic anhydride. Furthermore, by-products such as quinones are also oxidized further.
As a result of aging processes, all catalysts lose activity as the time for which they have been used increases. This occurs predominantly in the main reaction zone, since this is where the catalyst is subject to the highest temperatures. During the life of the catalyst, the main reaction zone migrates ever further into the catalyst bed. This steadily decreases the length of the remaining catalyst bed and adversely affects the after-reaction. As a consequence, intermediates and by-products can no longer be reacted completely and the product quality of the phthalic anhydride produced therefore deteriorates to an increasing extent. An aging process is particularly critical in the case of high feed loadings. Although the fall-off in the reaction and thus the deterioration in product quality can be countered by increasing the reaction temperature, for example by means of the salt bath temperature, but only to a temperature of about 400xc2x0 C., this temperature increase is always associated with a loss in yield.
DE-A 1793267 (GB-A 1274471) describes a process for preparing phthalic anhydride, in which the overall oxidative reaction is divided in process engineering terms into two parts. The reaction is controlled so that the reaction conditions in the second part, known as the after-reaction, are significantly more aggressive than in the first part. This can be achieved, for example, by carrying out the after-reaction without cooling, i.e. adiabatically. This after-reaction can be carried out in a separate reactor having different tube dimensions or even in a downstream shaft oven.
DE-A 2005969 describes a process for preparing phthalic anhydride, in which from about 80 to 99% of the total feed is reacted isothermally, i.e. cooled, in the main reaction. Conversion of the remaining unreacted feed occurs in a downstream adiabatic reactor. In addition, in the reaction procedure described, the gas mixture leaving the isothermal reactor is cooled further before it enters the downstream adiabatic reactor. This process variant is likewise intended to enable the phthalic anhydride formed to be obtained largely free of by-products and without a loss in yield. Here too, a shaft oven is claimed as adiabatic reactor.
Owing to the laminar flow occurring in honeycomb catalysts, they have only a very low pressure drop even at very high gas velocities. However, a disadvantage is that, owing to the lack of turbulent flow resulting from the shape, heat and mass transfer in the honeycomb channels, and thus heat removal, are greatly reduced. This situation makes use of honeycomb catalysts as catalyst supports virtually impossible for strongly exothermic processes in conjunction with a selective oxidation. Honeycomb catalysts have therefore become established industrially only in waste gas purification or waste gas incineration where all the organic constituents undergo total oxidation to CO2.
Coating monolithic support material with a catalytically active composition comprising the main constituents TiO2, V2O5 and possibly dopants by generally known methods, for example a dipping process, is found to be impractical. This is because coating suspensions based on commercially available TiO2 have a very high viscosity even at solids concentrations of 30-35% by weight and thus make coating of the channels of a monolithic support material virtually impossible without blocking the channels.
In order to coat monolithic catalyst supports with the necessary amount of catalytically active composition, for example 50-150 g of active composition per liter of catalyst, the coating process would have to be carried out with such a low-concentration xe2x80x9cactive compositionxe2x80x9d suspension that the necessary layer thickness would be achieved only after repeating the coating process a number of times. However, this at the same time once again increases the problem of blocking of the channels in the catalyst support because of the multiple coating steps. Furthermore, this is associated with significantly more work and thus with increased costs and is therefore uneconomical.
It is therefore an object of the invention to provide a simple and preferably single-stage process for producing monolithic catalysts based on TiO2/metal oxides.
It has now surprisingly been found that the viscosity of highly concentrated TiO2 coating suspensions having a high solids content can be greatly reduced by addition of surfactants
The invention provides a process for producing monolithic supported catalysts for gas-phase oxidation by coating the catalyst support by means of a suspension, wherein the latter comprises catalytically active composition comprising one or more types of TiO2 and 1-10% by weight of one or more surfactants of the formula
RnYmX
where R is the hydrophobic part(s) of the surfactant and n is 1, 2 or 3; Y is the hydrophilic part of the surfactant and m is 0, 1, 2 or 3, and X is the hydrophilic head group of the surfactant.
The viscosity of highly concentrated TiO2 coating suspensions having a solids content of greater than 30% by weight can be greatly reduced by addition of from 1 to 10% by weight, preferably from 2 to 5% by weight, of surfactants of the formula RnYmX.
In this formula, R is one or more hydrophobic parts, for example alkyl, aryl and alkylaryl groups, of a surfactant, where n is 1, 2 or 3, preferably 1 or 2. Y is the hydrophilic part of a surfactant, where m is 0, 1, 2 or 3, preferably from 0 to 2. X is the hydrophilic head group of the surfactant.
Preference is given to surfactants having head groups X selected from among phosphates, phosphonates, sulfates, sulfonates and carboxylates, dicarboxylates (malonic acid derivatives, succinic acid derivatives, adipic acid derivatives, maleic acid derivatives, phthalic acid derivatives) and polycarboxylates, for example polyacrylates, polymethacrylates or polymaleic acid derivatives substituted by surfactant radicals (R,Y).
In these head groups X, some of the acid radicals may be present in the H form as free acid groups, in the form of an ammonium salt or as a metal salt. Particular preference is given to free acid groups, ammonium salts and alkaline earth metal salts.
The hydrophilic group Y can be bound to the central atom of the head group X either directly or via an oxygen. Preferred central atoms are carbon, phosphorus and sulfur.
The hydrophobic groups R are preferably bound to the head group via a hydrophilic group Y.
Preferred embodiments of the hydrophobic parts R are alkyl radicals having relatively long-chain carbon building blocks with from 5 to 30 carbon atoms, preferably from 10 to 20 carbon atoms. The alkyl radicals can be saturated or unsaturated or branched carbon chains. The alkyl radicals can be bound directly or via aryl groups to the hydrophilic part Y or the head group X.
The hydrophilic radical Y generally comprises polymeric alkoxy units, preferably propoxy, ethoxy or methoxy units, with the degree of polymerization being able to be from 1 to 50 monomer units, preferably from 5 to 20 monomer units.
The coating suspension used according to the invention can comprise, for example, surfactants of the formula RnYmX selected from the group consisting of calcium alkylarylsulfonates., ammonium alkylarylsulfonates, calcium dodecylbenzenesulfonate, polyethoxy(dinonyl phenyl ether phosphate), polyoxoethylene(lauryl ether phosphate), polyethoxy(tridecyl ether phosphate), calcium dodecylbenzenesulfonate, tridecyl phosphate esters, ethoxylated phosphated alcohols, alkyl polyoxyethylene ether phosphate, ammonium nonyl phenyl ether sulfate.
The surfactants can be used without addition of further surfactants or together with other surfactants, for example alkylphenol ethoxylate.
The addition according to the invention of the surfactants to the coating suspension allows low-viscosity coating suspensions having high solids contents of TiO2 and/or V2O5 to be prepared and to be used for coating monolithic support material, for example honeycombs and supports having open or closed cross-channel structures. The coating suspensions may further comprise other additives, for example SiC. The solids contents of catalytically active composition in such suspensions can be set to values of up to 50% by weight and above. Such highly concentrated suspensions allow monolithic and, in particular, honeycomb catalyst supports coated with from 50 to 150 g of active composition per liter of honeycomb catalyst to be obtained without problems in one coating step.
Suspensions having a solids content of TiO2 of greater than 35% by weight have, owing to the high viscosity, greatly reduced flow and can therefore no longer flow through narrow channels. Changing to larger particle sizes does not lead to success either. The addition of one or more of the surfactants claimed significantly improves flow.
The catalysts of the invention can be produced using uniform TiO2 grades or mixtures of various TiO2 grades, which may in turn be doped or coated with metal oxides. The active composition preferably comprises V2O5 as additional component.
The coating of honeycombs with coating suspensions without addition of surfactants can be carried out without problems only using suspensions having a relatively low solids content of about 30% by weight. However, the amounts of active composition which can be applied in this way are only about 20 g/l of catalyst. If the solids content is slightly increased, the viscosity of the suspension increases so much that the suspension can no longer flow out of the honeycomb channels and blocking of the channels therefore results.
The use of the surfactants claimed enables the honeycombs to be coated without problems even using suspensions containing more than 50% by weight of active composition.
Applied amounts of over 100 g of solid/l of honeycomb catalyst can be achieved without problems in one coating step when using the surfactants claimed.
Examples of support materials suitable for coating by the process of the invention are materials such as cordierite, silicates, silicon dioxide, silicon carbide, aluminum oxide, aluminates or mixtures of these materials and metals or metal alloys. The support bodies can also have closed or open cross-channel structures. The suspensions used according to the invention enable honeycombs having a high to very high cell density to be coated without the danger of blocking the channels.
Preference is given to honeycombs having a cell density, i.e. a number of channels, of from 100 to 400 csi (cells per square inch), particularly preferably from 100 to 200 csi.
Monolithic catalysts are very well suited to the selective oxidation of o-xylene/air mixtures having low o-xylene contents to give PA. The monolithic catalysts do not in any event have a tendency to produce a runaway reaction. Surprisingly, the monolithic catalysts are superior to the conventional ring catalyst (for the same active composition).
Catalysts produced according to the invention and having a content of active composition of from 40 to 200 g per liter of catalyst are particularly advantageous. At a comparable temperature, these achieve higher conversions, better PA selectivities and smaller amounts of by-products.
The honeycomb catalysts produced according to the invention are very useful as catalysts for an after-reaction of a PA process gas comprising one or more of the starting materials o-xylene and naphthalene and/or intermediates such as tolualdehyde, phthalide, naphthoquinone, etc. This reaction is advantageously carried out at lower gas inlet temperatures, based on the temperature of the main reactor. In this after-reaction, a major part of the underoxidation products can be removed from the reaction gas and reacted further to form PA. Surprisingly, this also occurs at relatively high space velocities of 20,000-30,000 hxe2x88x921. Even in the presence of relatively high contents of underoxidation products together with a high concentration of PA, no runaway reaction occurs when using the catalysts of the invention.
The monolithic catalysts produced according to the invention are particularly suitable for preparing phthalic anhydride in an adiabatic reactor (after-reactor) in combination with an isothermally operated reactor (main reactor, for example filled with a bed of particulate catalyst).
The adiabatic reactor can also be operated advantageously with upstream gas cooling. In a particularly preferred embodiment, the upstream gas cooling and the adiabatic reaction are carried out in a joint apparatus.
In industry, it is customary to cool the reaction gas in a gas cooler before isolation of the product. The upstream gas cooling, the adiabatic reaction in the monolithic catalyst bed and further cooling can be carried out within the reactor or,outside the reactor, or in a joint apparatus.